Abstract

This thesis will investigate different methods for realizing terahertz (THz) radiation. The work will look at the current state of the art in technologies for generating THz radiation using two types of semiconductor laser diodes, the vertical-cavity surface-emitting laser (VCSEL), and the vertical-external-cavity surface-emitting laser (VECSEL).
The work starts by looking at designing a dual-wavelength laser inspired by work presented in the literature; with particular emphasis on reducing the wavelength spacing between the two wavelengths and improving the positions of the quantum-wells (QW) in order to reduce the residual absorption in the QWs. This naturally leads on to investigations into the effects of linewidth, and linewidth enhancement, on the performance of the device. It is found that linewidth enhancement is not a limiting factor in the design of dual-wavelength lasers.
The thesis will then investigate the technique of injection locking, simulated by rate equations, in order to investigate, in detail, the various behavioural regions exhibited by such a scheme under varying injection rates and detuning frequencies. The scheme will consist of a two laser system approach, whereby both unidirectional and bidirectional injection locking will be investigated. The disadvantage to such a scheme is the fact that there will be a zero frequency separation between the lasers frequencies while operating in the locked condition, hence the injection locking scheme will provide a building block for a three laser locking system based on four-wave mixing (FWM).
The interest in injection locking has been proven to be of great interest in the world of optics, ever since the Dutch scientist, Christiaan Huygens, discovered the phenomenon while confined to bed with illness during the 17th century. Such an approach has shown to be durable and efficient in improving the spectral and dynamic performance of directly modulated laser diodes.
The scheme of injection locking will be utilized in order to build a system based on the FWM phenomenon with a nonzero frequency separation between the lasers’ frequencies. As with the injection locking scheme, the various behavioural patterns at varying injection rates and detuning frequencies will be thoroughly investigated. The resilience of the system to perturbations (modulation response) will then be investigated, and the performance of the three laser FWM system will be compared to that of an uncoupled laser, whereby the phasor difference between the first and the second laser is calculated. The amplitude of the resultant wave is then compared to the amplitude of the uncoupled wave in order to establish whether or not the three laser FWM system supresses any of the introduced perturbations. It is found that the more the injection rate is increased, the more the FWM system supresses the effect of the perturbations, where a maximum improvement of 44% over the uncoupled laser is observed. It is also found that the system shows the behaviour of a first order system in series with a second order system in its frequency response.
The contributions made in this thesis include a new dual-wavelength VECSEL structure design, whereby the wavelength spacing between the two wavelengths has been significantly reduced, and the locations of the QWs have been improved.
Also, a system has been modelled utilising the injection locking scheme, in order to produce a nonzero frequency difference between the coupled lasers. For the first time, a thorough investigation of the locking regions has been undertaken at varying injection rates and detuning frequencies, whereby the different behaviours exhibited by the system in each region has been explored. A detailed investigation on the resilience of this new system to introduced perturbations is also presented.